The measurement of process parameters or physical properties of a medium by field instruments is preferably performed by utilizing low power 4-20 milliamp (ma) two wire transmitters or an equivalent low power information transmittal system. The low power operation affords instruments which are intrinsically safe in hazardous conditions and minimize the amount of wiring required to install the measurement instrument. A number of measurements require the processing of wave signals in order to obtain the most accurate and reliable of results.
A reliable and accurate means of performing the signal processing is to use digital signal processing techniques which have become readily available in recent years with the advent of microprocessors and digital signal processors. One of the characteristics of devices which perform digital signal processing is the high power required to perform the required signal processing and the data intensive nature of digital signal processing operations. However, the high power requirements of digital signal processing are incompatible with low power field instrument transmitters.
For example, the measurement of the flow velocity of a fluid in a conduit can be accomplished by transmitting ultrasonic pulses through the flowing fluid as it travels through the conduit or pipe and measuring the transit time of these ultrasonic pulses. The direction of propagation of the ultrasonic pulse is arranged so that the transit time is measured in the direction of flow (downstream) where it is decreased by the flow and opposite the direction of flow (upstream) where it is increased by the flow. The transit time is measured in both directions and the difference between the upstream and down stream transit times is used to determine the flow velocity.
A number of different techniques have been employed to accurately measure the transit time of ultrasonic pulses across a fluid containing conduit. For a typical measurement technique, the sound velocity of a propagated ultrasonic pulse is much greater than the fluid velocity and, thus, the transit time of the ultrasonic pulse is only slightly changed by the fluid velocity, thereby requiring the use of very sensitive equipment capable of detecting such slight changes in ultrasonic pulse transit times between a transmitter and a receiver. Flow meter instrumentation manufacturers have continuously attempted to develop equipment and systems which provide increased accuracy in the measuring of the transit time and flow velocity of fluid passing through conduits.
Early schemes transmitted ultrasonic waveforms through the moving fluid and analyzed the received ultrasonic waveforms to determine the flow rate of the fluid. For example, U.S. Pat. No. 4,787,252, which issued Nov. 29, 1988, provides a flow meter that transmits a signal modulated with a particular code through a fluid or gas medium. Then, the flow meter correlates the received signal with its originating transmitted signal to produce a correlation function having a peak at a time equal to the propagation time. The correlation function is determined by correlating, i.e., lining up, the signal pattern of the received signal with the known pattern of the transmitted signal. This scheme provides a highly accurate determination of the arrival time of the transmitted signal despite the presence of noise.
The amount of energy consumption of a flow measurement apparatus or flow meter is of great concern to a user, particularly for a battery-powered system, a two wire 4-20 ma transmitter system, a low power digital field bus system or an equivalent low power communication scheme. The flow meter disclosed in U.S. Pat. No. 4,787,252 consumes an excessive amount of energy before correlating the received signal. This is because such a flow meter uses a fast analog-to-digital converter (ADC) to digitize the electrical analog signal received by the receiving transducer. The ADC then transmits the digitized data to a memory circuit and direct memory access (DMA) controller which consume additional power. Thereafter, the digitized data held in the memory circuit and DMA is transmitted to a correlator (i.e., a microprocessor) which is capable of multiplying all of the digitized data point against the number of pulses which were generated in order to determine the flow rate of the fluid medium passing through the conduit or pipe.
As such, the flow meter of U.S. Pat. No. 4,787,252 consumes substantial amounts of energy during the analog-to-digital conversion, memory circuit, DMA and correlation steps making it undesirable for many low power in the field applications. Moreover, a reasonably powerful microprocessor is required to perform the correlation operation when individual digitized points are continuously being sent to the microprocessor. For example, the pattern of the coded transmission signal could be defined by 50 numbers sampled from a 5 cycle pulse of a 1 MHz signal. The electrical received analog signal might be scanned over a time interval of 50 microseconds to assure that the pulse is received. The scanned signal is converted to 500 digitized signals via an ADC device before being sent to the microprocessor for correlation calculation to obtain the flow rate of the fluid or gas medium passing through a conduit or pipe. The correlation operation that takes place within the microprocessor involves 50 multiplications performed 500 times or 25000 multiplications. This correlation operation would not be a problem for high power microprocessors with ample memory and high speed. However, when attempting to conserve power for in the field applications, the correlation operation does present a problem since low power flow detectors necessitate a substantial lowering of the speed of the microprocessor operation, thereby causing the calculations to take an inordinate amount of time producing a flow rate for a specific period of time. The extensive correlation calculations required by conventional flow rate detectors are also not suitable for low power detectors since the reduced speed of the microprocessor contained therein would also not allow for real-time detection of the flow rate of the fluid or gas medium passing through the conduit or pipe. These correlation calculations would also use substantial amounts of an already limited energy supply, thus effecting the overall performance of the detector itself.
The present invention provides a measurement apparatus which avoids the high power digital signal processing means while retaining the accuracy and reliability of the signal process function of the conventional digitized detectors. That is, the present invention overcomes the large power requirements of the conventional detector discussed above by compressing the amount of digitized data needed to determine the arrival time of an electrical received analog signal and calculate the flow velocity, distance, or other properties of the fluid or gas medium in order to minimize energy consumption.
The present invention provides a measurement apparatus which is capable of performing the correlation operation while the electrical received signal is in analog form such that a single output peak is produced from the analog correlator, thereby substantially reducing the number of samples of the analog signal which must be converted to a digitized form by an analog-to-digital converter (ADC) disposed between the analog correlator and a microprocessor. As such, the microprocessor is only needed to calculate the correlation peak time position, the transit time, the flow velocity, fluid or gas level or other physical properties of a fluid or gas medium from the few digitized data points transmitted from the ADC rather than perform extensive multiplication of numerous digitized data points as required in conventional flow rate detectors.
The present invention provides a measurement apparatus which is capable of determining exactly the time interval containing the peak value of the correlation of the received analog signal so as to clearly to identify and extract the correlation signal during the identified time interval, hereinafter referred to as the peak interval. This novel measurement apparatus limits the amount of digital information required for accurate determination of the transit time of an ultrasonic signal in a flow measurement operation, thereby reducing the multiplication operations previously required to calculate the transit time, and enabling such accurate and full measurements to be determined at a very low power consumption.